You can use clean master to free up your space.
In particular, to delete your thumbnails, follow these steps:
- Install clean master on your phone.
- Open it and click on "start".
- Tap "advanced" on the right top.
- Wait for the app section to load, then tap "gallery".
- Tap "Gallery thumbnails".
- Select all items by checking the box on the top right.
- Tap "clean".
- Tap "clean" again in the confirmation box.
Please note that, after you have deleted your thumbnails, previews of the photos in the gallery will take longer to load the first time.